Deputy Prime Minister Nick Clegg backed Prince Charles on his LBC Radio phone-in, saying: "Prince Charles should be perfectly entitled to express views in the confidence that he's expressing them privately."

Video:Farage: EU Backed Putin Into Corner

But Labour (Co-op) MP Mike Gapes, who represents Ilford South, said the Prince "should abdicate" if he wants to make controversial statements.

He said on Twitter: "If Prince Charles wants to make controversial statements on national or international issues he should abdicate and stand for election.

"In constitutional monarchy, policy and diplomacy should be conducted by parliament and government. Monarchy should be seen and not heard."

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Gallery: Prince Charles' Outspoken Comments

Architecture - National Gallery: In 1984 he described extension plans as a "monstrous carbuncle on the face of a much-loved and elegant friend".

UKIP leader Nigel Farage, who has backed Mr Putin's anti-EU stance in the past, said: "Prince Charles has made those comments - I know some people feel that way about Putin.

"I think there's a difference. The difference is right from the very start Hitler was expansionist, and we haven't see very much evidence of that until now from Putin and arguably, what's happened in the Ukraine is because he's been poked with a stick by the rest of the world."

Charles and the Russian leader are due to meet next month when they attend the 70th anniversary of the D-Day landings in Normandy on June 6.

Video:Clegg Backs Charles Over Putin Row

Mr Putin has faced international anger over Russia's actions in neighbouring Ukraine, including the controversial annexation of Crimea.

In March, former US Secretary of State Hillary Clinton reportedly said Mr Putin's claim to be be defending ethnic Russians in Crimea was "what Hitler did back in the '30s".

She later backtracked, claiming she was not making a direct comparison but that Russia's behaviour was "reminiscent" of Germany in the build-up to the Second World War.

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Gallery: Charles And Camilla In Canada

The four-day visit to Canada began in Halifax and includes stops in Pictou, Nova Scotia, Prince Edward Island and Winnipeg. This image shows the Prince visiting the woodwork department at Holland College in Charlottetown, Canada.

Prince Charles has been known for speaking his mind on issues such as architecture and the environment, but he rarely makes his feelings known on diplomatic matters.

There is an ongoing legal battle over the publication of letters he has sent to politicians, with the attorney general concerned their release could compromise the Prince's neutrality and create constitutional problems.